1. Field of the Invention
This invention relates to an automatic focus controlling device utilizing a linear solid state image pick-up device.
2. Description of the Prior Art
An automatic focus controlling device for an image reading system or a TV camera utilizing a linear solid state image pick-up device generally comprises a linear solid state image pick-up device, a taking lens for forming an image of an object on an image pick-up face of the solid state image pick-up device, a servomechanism for moving the taking lens along the optical axis thereof, a driver means which generates pixell (picture cell) clock pulses with a predetermined frequency and drives the solid state image pick-up device in synchronization with the pixell clock pulses, a differentiating means for generating contrast signals (which has its maximum level when the image is most sharply focused by differentiating time series electric signals from the solid state image pick-up device made by reading the image of the object on the image pick-up face thereof, a comparator which compares the level of two peaks of the contrast signals with each other at two different positions of the taking lens when moved along the optical axis thereof by the servomechanism, and a control circuit for controlling the servomechanism according to a difference signal generated by the comparator based on the difference between the levels of the two peaks.
Said linear solid state image pick-up device is also called a linear image sensor and is defined as an image pick-up device comprising a photoelectric element of self-scanning type such as CCD (charge coupled device) or a photodiode array. For example, in an image reading system a solid state image pick-up device or a CCD of bilinear type is employed as the linear solid state image pick-up device in order to obtain a doubled resolution power.
The bilinear type CCD is formed of two sets of photosensitive elements having different integration zones and arranged in the inter-digital form.
However, the bilinear type CCD is disadvantageous in that spike noises are apt to appear in time series electric signals generated thereby and that difference in intensity is apt to be observed between odd numbered signal and even numbered signal of the time series electric signals due to the difference in the characteristics of the two sets of photosensitive elements. These disadvantages will adversely affect upon the comparator when detecting the peaks of the contrast signal. Thus, there is a possibility of miscontrol of focus.